| Nuclear fusion energy is the future ultimate solution to the energy crisis. Nuclear fusion reactor works with the complicated environment, and that put forward stringent requirements for materials in fusion reactors. The irradiation effects of the first wall structural materials are important for the research of nuclear fusion. In this thesis, We briefly discussed the irradiation effects of hydrogen atoms in the first wall materials, investigated the changes in the mechanical properties and average magnetic moment caused by different chromium content in the ferromagnetic BCC Fe-Cr binary alloys. Considering the important elements tungsten with small amount in low activation steels, a systematic study of effect of chromium and tungsten element to mechanical properties of Fe-Cr-W ternary alloy was carried out. We hope these theoretical results help us to understand the irradiation effects in the relevant materials, and the calculations of the mechanical properties of steels are helpful for determining suitable compositions for the low activation steels.Tungsten-based materials are used as the first wall materials in ITER. Hydrogen impurities were introduced via bombarding with the reaction plasma, which are important for the behavior and stability of the tungsten wall. Using the first-principles density functional theory and planewave pseudopotential technique, we have simulated the behaviors of hydrogen atoms inside the perfect tungsten bcc lattice. The binding energies for different interstitial sites were compared to determine the optimal trapping site for the hydrogen atom inside the tungsten host lattice. The diffusion barriers for hydrogen atom migration between nearby trapping sites and the interaction between two interstitial hydrogen atoms were also calculated. The implication of our theoretical results on the hydrogen diffusion and accumulation behavior was also discussed.Because of the excellent resistance to swelling and embrittlement under irradiation and good thermal and mechanical properties, reduced activation ferritic/martensitic steels (RAFM steels) are considered as the primary structural materials of future fusion reactors. RAFM steels are composed of the body centered cubic (BCC) Fe-Cr alloys (7-12 wt.% Cr) with some trace elements (such as, W, V, etc.). Amount of experimental and theoretical results indicate that the Cr element play a significantly role in the mechanical properties and irradiation effect of RAFM steels. As the primary structure materials in ITER fusion reactors, the mechanical properties of RAFM steels are important to the safety. Thus, it is necessary to investigate the fundemental mechanical properties of Fe-Cr alloys with different Cr content. Modelling Fe-Cr alloys as a random solid solution model, we employed the first-principles method to inverstigate the effect of Cr element to the lattice constants and the elastic properties of ferromagnetic bcc Fe1-xCrx (0=x=0.156) alloys. We found that the lattice parameters of Fe-Cr alloys are all larger than that of BCC pure Fe solid. The Young's modulus and shear modulus of Fe-Cr alloys rise non-monotonically with increasing Cr composition. All Fe-Cr alloys with diferent compositions exhibit the ductile behavior.The average magnetic moment per atom of Fe-Cr alloys decrease linearly with increasing Cr concentration.The basic composition of RAFM steels is Fe-Cr-W alloy. Using the exact muffin-tin orbitals (EMTO) method, combined with coherent potential approximation (CPA) and full charge density (FCD) technology, we investigated composition dependence of mechanical properties for Fe-Cr-W random alloys within the composition range of 7.8-10.0 wt.% of Cr and 1.0-2.0 wt.% of W. Bulk modulus B, shear modulus G, Young's modulus E, BIG ratio, and Poisson ratio v were discussed as functions of ternary composition. On the whole, the elastic moduli of Fe-Cr-W alloys increase with chromium content; the influence of tungsten is relatively weak.
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